1. Field of the Invention
The present invention relates to a display device having a dynamic focus voltage circuit and particularly to a display device suitable for a TV set with a large screen size, a display device of cathode ray tube for a wide angle deflection, and a display device of cathode ray tube with a thick neck diameter.
2. Description of the Prior Art
Generally in a cathode ray tube to be used for a TV set and others, the arrival distance of electron beam is different between the center position of the screen and the corner position thereof, so that even if the TV set is adjusted so as to obtain a best focus in the center position of the screen, the focus in the corner position becomes dull as it is. Therefore, the focus is corrected by applying a voltage to the focus electrode of the cathode ray tube according to the deflection position of electron beam by the dynamic focus voltage circuit and the electron beam is kept in the best focusing state overall the screen. A prior art relating to such a dynamic focus device is described in Japanese Patent Application Laid-Open 54-155721.
Since the screen of each display is becoming larger and the brightness thereof is increasing recently, a request to a dynamic focus circuit is becoming severe.
As a first problem, it is necessary to make this dynamic focus voltage extremely high due to a wide angle deflection of the cathode ray tube and a large diameter of the electron gun.
As a second problem, since a large mean beam current is set so as to realize a high brightness, a high voltage dynamic regulation occurs, and the focus voltage for the high voltage relatively fluctuates, and the best focusing state cannot be maintained. The second problem will be explained briefly by referring to FIG. 6. FIG. 6(a) shows the status of a white band pattern displayed on the screen. In the portion equivalent to the white band on the screen, the signal amplitude of the signal waveform shown in (b) is large. In correspondence with it, in the portion of white band of the high voltage waveform shown in (c), nearly 1/2 of the vertical cycle is required for the high voltage to drop and recover to the predetermined value. However, the voltage waveform at the focus terminal is not so large unlike the change in the high voltage as shown in (d), and the electron gun enters the overfocusing state, and the focus reduces over a wide area from the white window portion to the lower part of the screen.
To solve the first problem, a method of generating a high voltage using a boosting transformer and a method of generating a high voltage using active elements such as a high voltage supply and a transistor are used. Each of them has its merits and demerits and the problems indicated below are imposed.
(1) A case of a boosting system using a transformer PA1 (2) A case that an active element such as a transistor is used
When the boosting ratio of a transformer is set high, a sufficient voltage amplitude is obtained. However, since the core size of the transformer is increased, there are disadvantages such that the area of the board is required to increase and the materials of the transformer are expensive.
To obtain a high voltage amplitude, it is necessary to increase the supply voltage, increase the withstand voltage of the active element, and reduce the load resistance so as to preserve the slewing rate. As a result, there are disadvantages such that the power consumption of the whole circuits increases extremely, and the area of the board increases because a heat radiation board is required, and the materials are expensive including high withstand voltage elements.
For the second problem, it is necessary to suppress a dynamic regulation in the high voltage. For example, it is necessary to use a high voltage stabilizing circuit and a deflection high voltage separation system. However, in both cases, a problem arises that the number of parts increases and the materials are expensive.